(1/2)x^2+xy+(1/2)y^2
=(1/2)(x^2+y^2)+xy
=(1/2)[(x+y)^2-2xy]+xy
=(x+y)^2/2-xy+xy
=(x+y)^2/2
=1/2=(x+y)^2/2-xy+xy=(x+y)^2/2 有点看不懂，能不能麻烦用一点汉字。(1/2)[(x+y)^2-2xy]+xy把前面的1/2乘到中括号里面